The invention relates to a cutting insert, for example, for a milling tool, particularly a face-milling cutter.
Milling tools are often provided with cutting inserts made of hard metal or other hard material optimized concerning their cutting properties. On the cutting inserts cutting edges, clearance surfaces and rake surfaces are formed, whose shape significantly affects the performance of the cutting insert tool. At the same time, consideration is given to the chip flow, that is, to the manner in which the chips are formed and how they run over the rake surface which adjoins the cutting edge.
It has been found that during chip breaking some materials tend to leave deposits on surfaces over which they run. Such an occurrence is referred to as “smear-on”. A material accumulation occurring by smear-on may be disadvantageous for several reasons. If, for example, the cutting insert is provided with a wear-reducing coating, the latter may be endangered by a material accumulation. If the material accumulation grows excessively large and is entrained by the chips sweeping thereover, under certain circumstances damage to the coating may occur. Further, material may accumulate on surfaces or surface regions which, in a different orientation of an invertible cutting insert, may serve as indexing or supporting surfaces. A material accumulation on such surfaces thus results in a faulty positioning which is to be avoided.
German patent document DE 196 53 921 A1 describes a triangular invertible cutting insert which has an all-around extending cutting edge joined by a rake tray, that is, a tray-shaped rake surface. The rake surface changes into a top surface which lies below the cutting edge. The top surface is, nevertheless, situated in the region of chip flow, so that at such a location material accumulation may occur.
The above-noted German patent document further discloses, in top plan view, four rhomboidal cutting inserts having an all-around extending cutting edge. Again, a rake surface joins the cutting edge at a positive rake angle. In the obtuse corner regions of the rhomboidal insert the rake surface changes in a stepless manner into a planar supporting surface which lies below the cutting edges. In the acute-angle corners, the rake surface, which is immediately adjacent the corner cutting edge, is adjoined by a plateau which, in turn, changes into the supporting surface with a step. The outwardly concave, obtuse angle between the rake surface and the plateau causes the chips to run against the plateau surface where they may form deposits.
British Patent 1,154,754 discloses a lathe tool including a cutting insert which has a square-shaped base outline and four side surfaces arranged at right angles to one another. The side surfaces border cutting edges arranged as a square. The cutting edges are adjoined by rake surfaces which are raised with respect to a planar top surface of the cutting insert and are thus separated from the top surface by an all-around extending step. The latter extends along the rake surfaces and also beyond the corner regions and has a constant height along its entire periphery.
Accordingly, it is an object of the invention to provide a cutting insert which has a reduced tendency to allow material deposits to form thereon during chip breaking.